A flat washer is a thin, round plate that sits under a bolt head or nut. It spreads the clamping load over a wider area and protects the joint surface.

Quick answer: washer size follows bolt size. An M10 washer has a 10.5 mm inside diameter, a 20 mm outside diameter, and a 2.0 mm thickness.

This washer size chart lists ISO 7089 washer dimensions for metric bolts. It also covers USS and SAE washer sizes in inches, so you can match the right washer to the right bolt.

ISO 7089 Flat Washer Size Chart (M1.6–M64)

ISO 7089 covers plain washers, normal series, product grade A. It applies to nominal sizes from 1.6 mm to 64 mm, in two hardness classes: 200 HV and 300 HV. HV stands for Vickers hardness.

DIN 125 Form A is the older German standard for the same washer pattern. Most suppliers treat the two as equal. On critical joints, check the drawing for tolerances.

Sizes such as M3.5, M7, M18, M22, and M27 also exist. They sit between the sizes above.

Two patterns help when you scan the chart:

• The outside diameter is roughly twice the bolt diameter for common sizes.
• The washer thickness grows from 0.3 mm at M1.6 to 9 mm at M64.

The hardness class matters too. The standard says 200 HV washers suit bolts up to property class 8.8 and nuts up to class 8. 300 HV washers suit bolts up to class 10.9.

Other washer standards share the same ISO family:

• ISO 7090: chamfered washers.
• ISO 7091: product grade C, a rougher finish.
• ISO 7093: large series, with an outside diameter of about three times the thread diameter.

Flat Washer Size Chart in Inches: USS vs SAE

US shops often use inch washers. The two common types are USS and SAE. Both fall under ASME B18.22.1, which covers Type A and Type B plain washers.

USS washers are the wide series. They have a larger outside diameter and spread the load further. SAE washers are the narrow series. They have a tighter fit and a thinner body.

Values can differ slightly between suppliers and standard editions. Check your drawing for critical work.

Here is a rough match between metric and inch bolts:

• M6 is close to 1/4″.
• M10 is close to 3/8″.
• M12 is close to 1/2″.
• M20 is close to 3/4″.

Do not swap metric and inch washers on the same joint. The hole may look right but fit loosely or tightly.

How Washer Size Is Measured

Every flat washer has three washer dimensions:

• Inside diameter (ID): the hole size.
• Outside diameter (OD): the full width of the washer.
• Thickness: the height of the plate.

Use digital calipers for the best reading. Measure the ID across the hole, the OD across the widest point, and the thickness at the edge.

Here is the part many buyers miss. A washer is named for the bolt it fits, not for its own hole. So an M6 washer is made for an M6 bolt. Its hole is slightly bigger than 6 mm, which gives the bolt clearance.

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How to Choose the Right Washer Size and Material

Start with the bolt size. Then pick the washer thickness and outside diameter for the job.

• Standard joints: use the normal series, such as ISO 7089.
• Soft materials or oversized holes: use a large series washer. ISO 7089 itself says to check suitability in these cases.
• High-strength bolts: use the 300 HV class for bolts up to property class 10.9.

Material matters just as much:

• Zinc plated steel: good for dry, general work.
• Stainless steel A2: suits damp or corrosive places.
• Stainless steel A4: the usual pick for marine and chloride exposure, such as coastal or salt-spray sites.

Avoid these common mistakes:

• Mixing USS and SAE washers on one job.
• Using a metric washer on an inch bolt.
• Ignoring thickness, which affects how well the washer carries load.

Need a non-standard size? Send your drawing to a fastener supplier. Many quote special washers in about a day.

FAQs

What is the standard dimension of a washer?

There is no single size. Washer dimensions depend on the bolt size and the standard. For example, an ISO 7089 washer for M10 is 10.5 mm ID, 20 mm OD, and 2.0 mm thick.

What are the dimensions of an M20 washer?

An M20 flat washer has a 21 mm inside diameter, a 37 mm outside diameter, and a 3.0 mm thickness.

How is washer size measured?

By inside diameter, outside diameter, and thickness. The washer is then named for the bolt size it fits.

What size is a 12mm washer?

Most people mean a washer for an M12 bolt. It has a 13 mm ID, a 24 mm OD, and a 2.5 mm thickness.

What are the dimensions of an M10 washer?

An M10 washer has a 10.5 mm ID, a 20 mm OD, and a 2.0 mm thickness.

Is M6 washer 6mm?

No. M6 names the bolt, not the washer. An M6 washer has a 6.4 mm ID, a 12 mm OD, and a 1.6 mm thickness.

Conclusion

Pick the washer by bolt size, then check the ID, OD, and thickness against the chart. Match the standard, the hardness class, and the material to the job. That keeps the joint tight and the surface safe.

Timmy Ahad

Measurement Researcher, Size Comparison Writer & Content Specialist
Timmy Ahad is a dedicated content writer and research specialist at SizesVisualizer.com, where he focuses on creating accurate, easy-to-understand content about measurements, dimensions, size comparisons, and real-world object visualization.